The present invention relates to a contactless IC card for receiving power and for receiving/transmitting signals from/to a reader/writer as an upper terminal by electromagnetic coupling. More particularly, the invention relates to the structure of an antenna coil in the contactless IC card.
In recent years, attention has been paid to an IC card having therein an IC such as a processor as a storage medium of electronic money or the like. There are two types of IC cards according to different signal transmitting methods. One is a contact IC card which receives power and clocks from a reader/writer as an upper terminal by using a contact point and processes a command from the reader/writer. The other is a contactless IC card 32 in which, as shown in FIG. 3, a coil 321 receives radio waves (electromagnetic waves) generated from a reader/writer 31 through a coil 311, transmitting and receiving data signals are generated, and a command transmitted from the reader/writer 31 is processed.
The contactless IC card includes a contact type having the communication distance of about few mm to 10 mm, a proximity type having the communication distance of tens mm, and a remote type having the communication distance of tens cm to few m. Among them, the contact IC card has the international standard ISO10536-2 in which the location and shape of the coils and the electric characteristics are defined.
According to the standard ISO10536-2, as shown in FIG. 3, page 2, ISO/IEC 10536-2:1995(E), two coils are arranged at an interval of 22 mm in a card and the inner size of the coils is defined as (11.6 mm.times.4 mm).
In order to reduce thickness, an IC card in which antenna coils are formed by printing a conductive paste (for example, silver paste) or the like on a PET film is used. As conventional techniques of the IC card obtained by printing the conductive paste, there are techniques described in Japanese Patent Application Laid-Open No. 8-216570, Japanese Patent Application No. 7-120237, and the like.
The method of forming the antenna coil of the IC card by printing the conductive paste (for example, a silver paste) or the like on a PET (polyethylene terephthalate) film of the conventional techniques have a problem that the conductance, that is, the value of resistance of the coil is larger than that of a coil made of a general copper wire and that of an etching coil on a printed board by one digit or more.
Specifically, in a coil having about 20 turns, although a direct current resistance of a coil formed by copper foil etching is 10 .OMEGA. or less, the resistance of a thick film printed coil formed by printing the silver paste or the like is 100 to 200 .OMEGA.. When power is received and a current flows, if the direct current resistance of the coil is large, the coil itself consumes the power. Consequently, a loss occurs and power efficiency deteriorates. It is therefore important to reduce the resistance of the thick film printed coil by widening the pattern line width of the coil as much as possible.
The coil is printed on the PET film in order to reduce the thickness of the card in the conventional techniques. When coils are printed on both of the surface and rear surface in order to increase the number of turns of the coils, there is a problem such that the antenna coils themselves have a self resonant frequency by a floating capacity (capacitor) occurring between the coils on the surface and rear surface and L (reactance) of the coils and an adverse influence may be exerted on a transmission frequency band. Specifically, the self resonant frequency by the floating capacity when the silver paste coils each having about 20 turns are formed on the PET film having the thickness of 50 .mu.m is few MHz to tens MHz. Since the power and the communication frequency of the contact IC card conforming to ISO10536 are defined as 4.91 MHz, the self resonant frequency is a problem.